This invention relates to a pre-cure resistant liquid phenol-aldehyde condensation resin which are useful as adhesive and binders in waferboard manufacture.
In the present manufacture of waferboard, green wood wafers, which have been dried to 3-6% moisture content, are sprayed with molten wax, about 2%, based on dry wood weight; and powdered phenolic resin of 2-3 wt. percent is blown into a rotating blender. The resulting wax and resin-coated wafers are felted on a warm caul plate which is recycled through a hot-press. In mill practice, the warm caul plates are used directly without cooling, a recycle temperature of about 70.degree.-110.degree. C. The resulting loosely formed wafer mats are then hot-pressed to consolidate the board and cure the powdered resin binder. The maximum pressure of the hot press is about 450-500 pounds per square inch (31.5-35 kg/cm.sup.2) and the hot press closing time is usually 1-2 minutes, depending upon board thickness desired, due to the high rigidity of the dry wood wafer mat. The surface of the mats are subjected to high temperature, i.e., 190.degree.-210.degree. C., during the loading and hot-press closing stage.
Powdered phenolic resins such as novolac, resole or combinations thereof, are generally used in present waferboard manufacture since they easily blend into the wafers for uniform resin distribution and they are pre-cure resistant and long storage life. U.S. Pat. No. 4,098,770 discloses a typical spray-dried powdered phenol-formaldehyde resin, modified with added non-phenolic polyhydroxy compound, which is used for waferboard manufacture. On the other hand, powdered resin has the disadvantage of being (1) expensive to produce, (2) requires a high wax content (about 2%) to improve affinity of resin powder on the wafer surface and (3) contribution to mill dust which in turn causes air pollution and health and safety, e.g., explosive hazard.
Liquid phenolic resins have been used in particle-board and plywood manufacture, but this application to waferboard, by an airless spray system, has been found to be ineffective due to problems of pre-cure and large resin droplet size. Pre-cure occurs before the waferboard is completely solidified, so that board surface layers are considerably weak. The geometrical shape of wood wafer also does not allow transfer from resin-excessive wafers to resin-deficient wafer by the rubbing effect used with wood particle.
An efficient waferboard liquid resin must therefore be pre-cure resistant and be able to be atomized in fine droplets to maximize wafer surface coverage. Thus, low viscosity and low surface tension are essential properties of the waferboard resin.
U.S. Pat. No. 3,267,188 disclosed a process for forming a composite board using post heat treatment to completely cure a binary resin binder consisting of a green phenolic resin and an advanced phenolic resin. The green phenolic resin is used to impregnate the wet wood particles and the high molecular resin is retained on the wood particle surface to serve as a binder after the resin treated wood is oven dried. The dissimilar character and geometrical shape of dry wood wafer precludes the disclosed impregnation of a green phenolic resin and does not suggest application by spraying in fine droplets on a dry wood wafer.
Other aqueous alkaline phenol-formaldehyde resins have been proposed as adhesive formulations for plywood and hardboard, such as that disclosed in U.S. Pat. No. 3,342,776, wherein a highly methylolated phenol is first produced under conditions which "prevent the formation of condensation but reduces free-formaldehyde" level and then further reacts at reflux temperatures to produce a highly reactive resin which, still has a "low enough molecular weight to provide for penetration of the wooden adherends". The resin, though of lower viscosity, lacks the pre-cure resistance necessary for waferboard application.
U.S. Pat. No. 3,591,535 discloses a liquid phenol-formaldehyde resin for hardboard which has a low molecular weight by combining a low viscosity resin with a resin which has been advanced to a high viscosity. This resin also lacks pre-cure resistance and has a viscosity which is still too high, i.e., 600-800 cps at 70.degree.F. to be suitable for waferboard application. U.S. Pat. No. 3,839,251 also discloses a lower viscosity advanced phenolic resin for particleboard which is prepared by a two stage condensation reaction. The resin lacks pre-cure resistant properties and has a viscosity and surface tension considerably higher than that required for waferboard application.
U.S. Pat. No. 3,927,140 discloses an aldehyde condensation copolymer formed by co-condensing linear and non-crosslinkable aldehyde prepolymer with a highly thermosetable and cross-linkable prepolymer. Improved resin of flakeboard made from mixed hardwoods have been suggested by formulating phenolic resin with a second formaldehyde additive near the end of a conventional phenolic resin cook, as shown in Formulation of An Economical Fast-Cure Phenolic Resin For Exterior Hardwood Flakeboards by Chung-Yun Hse (Proceedings of Ninth Washington State University Symposium on Particleboard, April, 1975, Pullman, Washington).
These resins also do not contain significant amounts of non-resinous phenol-formaldehyde condensates to impart pre-cure resistant properties and have viscosity and surface tension significantly higher than that required for atomization on waferboard.